Across the counties surveys are finding a huge variety of types of graffiti, much of which can be grouped into themes.
Crosses can be found almost anywhere in churches. There are a number of explanations, the most frequent that they are 'pilgrims' crosses' - inscriptions made during a pilgrimage. This may be relevant for those churches on pilgrimage routes, but crosses can be found on churches in any location. It may be that inscribing a cross is the simplest way to express one's religious devotion.
The crosses we find are being described in heraldic terms for ease of identification, and does not mean that an association with heraldry is being made.